Question: In geometry class, the girl to boy ratio is $9$ to $4$. If there are $45$ girls, how many boys are there?
A ratio of $9$ girls to $4$ boys means that there is a group of $9$ girls for every group of $4$ boys. If there are $45$ girls, then there are $5$ groups of $9$ girls each. Because we know the ratio is $9:4$ , there must be $5$ groups of $4$ boys each. There is a total of $20$ boys in geometry class.